The laid-open publication DE 199 38 274 A1 discloses, in this respect, a method and a device for the controlled setting of the radial gap between stator and rotor arrangements in a gas turbine. The design-related radial gaps are formed between the rotatable moving blades of the rotor of the turbomachine and the guide faces lying opposite them fixedly in terms of rotation on the stator. The guide faces serve for guiding the working medium and are formed by annular segments which are subdivided in the circumferential direction and extend coaxially as a guide ring about the axis of rotation of the rotor in the axial direction. When the gas turbine is in operation, the moving blades of the rotor move at a distance from the guide faces. Conversely, free-standing guide blades can also in each case form a radial gap with respect to a rotating conical or cylindrical guide face arranged on the rotor. In order further to optimize the efficiency of the gas turbine, the radial gaps are to be designed so as to be as small as possible. It is known from the abovementioned laid-open publication to fasten guide rings to a stator by means of holding partners arranged obliquely with respect to the radial direction and, during the operation of the gas turbine, to displace this stator in the direction of the moving blade ends by virtue of the thermal expansion of the material of the guide ring, in order to make the radial gap smaller.
Something similar is disclosed in EP 1 163 430 B1. During the operation of a gas turbine, a guide element lying opposite a tip of a turbine moving blade flexes in the direction of the moving blade tip on account of the thermal expansions, thereby making the radial gaps smaller. At the same time, the guide element can be acted upon with cooling air from the rear side, so that it can withstand the temperatures prevailing in the flow duct.
Moreover, it is known from GB 2 397 102 A to insulate the guide ring of a turbine with respect to the carrying structure.
It is known, furthermore, that the design parameters determining the gap dimension are designed for the hot starting of a gas turbine, in order to satisfy the requirements for the smallest possible operating gap, that is to say radial gap. After the gas turbine has been run down, the casing cools comparatively quickly, as compared with the rotor of the gas turbine. The casing or the guide rings, on account of their cooling, shrink back to their original design size, the still hot rotor initially remaining expanded due to the heat stored in it and cooling and shrinking with a delay. This gives rise to what is known as the contraction effect. This situation may result in the radial gap decreasing as the blades of the rotor touching or even brushing against the casing or the guide ring, thus permanently enlarging the radial gap or possibly even damaging the blade. An enlarged radial gap leads to increased fuel consumption, and damaged blades may make it necessary to carry out premature maintenance with the corresponding extra costs.
During the starting, that is to say run-up, of the gas turbine, the centrifugal forces acting on the moving blades bring about a further expansion of these, which may close the radial gap still present before the starting of the gas turbine and cause a harmful and unwanted brushing against of the blades.